An important application for high power amplifiers in the current art is in the area of wideband cellular communications (e.g., cell phones and cell systems including base stations in communication with the cell phones and providing connection to the telephony system). For example, a typical application for a high power amplifier is in implementing the transmitter function for a cellular telephone base station (of course, the base station receives as well, however the receiver functionality is not addressed here). FIG. 1 depicts a base station circuit 10 in an illustrative prior art cellular communications system. Base station circuit 10 is coupled to an antenna 18 for transceiving signals to a variety of communication units 12, 14 and 16. These communication units are depicted as handheld portable cellular telephones, although these are merely examples and many other devices are presently communicating over these systems. In a typical system the cell phones may communicate their availability to the base station that is nearest to the phone at a given time, or, that has the most capacity for adding additional communication channels, using well known industry standard defined signaling including registration, hand-off, and seeking techniques. If the cell phone moves from one location to another, various handoff and registration techniques are used to establish communication with the next nearest or most available cellular base station and antenna, as is known in the art. The cell phones can simultaneously send and receive communications including digitized voice and digital data to the base station circuit 10 using, for example, wideband spread spectrum signaling over the air via the antenna 18. This is accomplished using several well known standard protocols for voice and/or data transmission, for example CDMA, WCDMA, TDMA, GSM, EDGE, 3G and the like.
Although the example in FIG. 1 depicts the units 12, 14, 16 as portable cell phones, it is known to incorporate many other features that require substantial data bandwidth. Examples now implemented, or planned in the near future, include video streams, audio streams, email, data file transfer, text messaging, instant messaging, mobile internet access and mobile television viewing, mobile payment systems, all applications included in certain advanced or announced cell phones. These applications will continue to require increasing amounts of data to be transmitted over the cellular system. Further, the communications units may be devices other than cell phones, including wireless portable email terminals, computers both fixed and portable such as laptops and palm computers, fixed location, handheld and vehicle mounted telephone equipment, personal internet browsing devices, even video equipment and other communications or data receiver or transmitter applications. The base station circuitry 50 provides a link between the conventional wired telephone network (not shown), which may include switches, copper and fiber optical cables and equipment, and repeaters, concentrators and routers as is known in the telephony art, to the cellular system.
FIG. 2 depicts a simplified schematic of the RF transmit portion for such a base station application. (Only a single RF transmit function and one antenna is shown, however, many may be used). In FIG. 2, digital baseband transmit data Vk is input to a digital to analog converter (DAC) 3, which outputs analog data for transmission to a frequency converter 5, clocked by a local oscillator 15, the frequency converted analog signal is next presented to a power amplifier 11, and the resulting analog amplified output signal Vo is presented to an antenna 12 by a coupler 13 for transmission. (In this example schematic, a single amplifier and a single antenna are shown, but in a practical commercial system many amplifiers and many antennas may be coupled in parallel at a single location). A feedback path then returns an observed version of the transmitted signal Vo for observation. This is accomplished, for example, by coupling the antenna to a second frequency converter 17 which is likewise clocked by local oscillator 15, the frequency converted analog signal is then processed by analog-to-digital converter 19 and digital signals Vfb are made available for observation, and for use in compensation of the transmit signal using conventional feedback techniques.
Generally, high power amplifiers as are used in the communication art exhibit non-linear output characteristics, particularly as the transmit power level is increased. This non-linear behavior results in distortion in the output signal that is undesirable, and may cause the communications system to fail to meet the required performance metrics as set by standards organizations, or the governing regulatory agencies. However, as is well known, any power transistor driving a load will act in a nonlinear fashion when operating in the transistor saturation region, that is, the linear relationship of the voltage input-voltage output characteristic is only maintained in the linear operation region of the transistor. As the input signal power is increased to and beyond the saturation point of the driving transistors in the power amplifier, the response of the transistor(s) does not change linearly and the power output characteristic therefore inherently becomes non-linear.
A simple and somewhat effective prior art approach to limiting non-linear power amplifier distortion for base station transmitters is to simply “back off” the output power, so that the power amplifier operates only within the linear part of its voltage response characteristic. However, to be effective this output power “back off” must be sufficient to account for the peak-to-average ratio (PAR) of the input signal, and the required transmit power back off to maintain linear performance of the driving transistors over the entire range of input signal conditions may therefore become very significant. This increased back off results in very low amplifier efficiencies. As an example, a CDMA signal of the present cell phone systems may exhibit a PAR of up to 13 dB. Power back off is a well known source of inefficiency in the operation of the high power amplifier in a transmitter, because the amplifier is being intentionally operated at far lower than possible output power levels, for a given supply power. This inefficiency results in additional power consumption during operation of the system and therefore increases the manufacturing costs of the system, because to achieve a certain output power requires that a much more powerful amplifier be used, while if the back off were not required, a lower power amplifier can be used, and lower costs for the amplifier would therefore accrue. This inefficiency causes increased operating or ownership costs for the system, as the operating power expended for the actual transmit power achieved is increased.
A characteristic of modern communications systems is the use of linear signal modulation such as quadrature phase shift keying (QPSK), quadrature amplitude modulation (QAM) and multicarrier configurations. Signals for transmission currently combine multiple signal channels and exhibit a fluctuating signal envelope of combined RF carriers that are to be transmitted at a single transmitter. These characteristics increase demand on the performance of the power amplifier over the earlier analog cellular and PCS cellular systems, which had multiple power amplifiers driving a signal combiner and antenna. In the current systems, intermodulation distortion (IMD) occurs and due to the adjacent channel leakage introduces adjacent channel interference (ACI) in the adjacent channel receivers. These added pulses/signals introduce distortion, which reduces the overall system performance and the bit error rate. However, the degradation that results are overcome with the additional transmit power that is possible. A highly linear power amplifier is required to transmit acceptable quality signals in these systems. The governing standards for various systems may specify minimum performance criteria, one of which may be expressed, for example, as a required Adjacent Channel Leakage Ratio (ACLR). For a current WCDMA standard base station, the 3G standard specification requires an ACLR performance of 45 dB in the adjacent channel. This metric is a measure of the transmit power detected in a non-used channel when there is transmission in an adjacent channel. The more linear the components in the power amplifier used for the system are, the better the ACLR performance will be, and also the more efficient the operation of the power amplifier may be, with reduced need for backoff.
The term “crest factor” is sometimes used in the art for describing the significance of the peak amplitude verses the average amplitude of a particular signal. One prior art approach to reducing the amount of “back off” required to maintain the amplifier in its linear operating range is to modify the input signal prior to applying it to the power amplifier for transmission. This signal modification is accomplished by applying a technique called “crest factor reduction” or “CFR”. CFR changes the input signal prior to presentation to the high power amplifier by removing the “crests” in the input signal. In one known approach a cancellation signal is applied. As large amplitude peaks over a defined threshold are identified in the input signal, a compensating signal with an opposing crest is created and then this inverted signal is combined with the input signal so as to cancel only the largest peaks in the input signal, and so reduce the crest factor, i.e. reduce the PAR, prior to presenting the signal to the power amplifier for transmission. This approach advantageously reduces the amount of “back off” required to maintain the power amplifier in its linear range, and efficiency for the system is thus increased.
Commercial processors are available that are specifically designed to provide the CFR function, for example Texas Instruments, Inc. supplies integrated circuits designated as part no. GC1115 particularly directed to implementing CFR in cellular communications systems. The GC1115 is a crest factor reduction preprocessor that receives digital upconverted input signals as I (in phase) and Q (quadrature phase) form digital signals, for example, and outputs a modified I, Q, signal with the peaks removed for transmission with reduced PAR. The GC1115 integrated circuit is operated by programmable DSP executing software provided by the user.
FIG. 3 depicts a simplified block diagram of some of the functional blocks of the prior art GC1115 processor. There are, coupled in series cascade fashion, four individual peak detection and cancellation (PDC) blocks 30. Each one of these PDC blocks is coupled to a group selectably provided from the thirty-two included pulse cancellation generators in block 34. Phase lock loop circuits PLL CORE 36 and PLL TX 38 are provided to provide timing signals. The block labeled MICROPROCESSOR INTERFACE 39 includes control registers, storage RAM, synchronizers and other supporting circuitry required to enable a user programmable device, such as a DSP, ARM, RISC or general purpose microprocessor, to control the GC1115 according to user defined programmation. The block labeled 37 receives the I, Q output data from the CFR processor and includes the many functions that typically follow as shown, for example in FIG. 2. In operation, the GC1115 PDC blocks 30 are used to detect and cancel peaks in the input signal, which is input in the form of I and Q phase sampled digital signals or vectors. The cancellation peak generators in block 34 are used in conjunction with PDC stages to remove the peaks. Typically, coarse peaks are removed at the first stage, and finer and finer peak cancellation is performed by each subsequent pipelined stage, until the peaks are removed to achieve a desired PAR reduction.
FIG. 4 depicts the use of the CFR GC1115 processor in an exemplary prior art system for transmitting baseband signals. In FIG. 4, signals are provided to the cellular station for transmission and are input to a digital upconverter DUC 40. CFR processor 42 receives the upconverter output and provides corresponding output signals with the reduced peaks desired, for improving amplifier efficiency in the power amplifier stage without the need for excessive back off. Digital to analog conversion is performed in D/A converter 44 and the analog signals are upconverted to an appropriate radio frequency by the function RF upconverter 46. Power amplifier 48 receives the signals and transmits them via the antenna 50. DSP or microprocessor 52 controls the operation of the CFR processor 42.
While the approaches of the prior art can maintain the high power amplifier in a more linear operating range by reducing the crest factors of the input signal, these approaches require a modification of the input signal and thus distort the signal somewhat as a necessary consequence.
Thus, there is a continuing and increasing need in the art for a system and method that provides efficient adaptive linearization of the power amplifier through crest factor reduction alone or in combination with improved predistortion functions. The system should be realizable using commercially available technology, and compatible with and useful in conjunction with power amplifier implementations that include, or alternatively that do not include, digital predistortion circuitry to further enhance the performance of the power amplifier. Embodiments and methods of the present invention address this need.